1. Technical Field
The present invention relates to a drum type washing-drying machine in which a heat pump mechanism is used to dry clothes in a drum.
2. Related Art
General drum type washing-drying machines are constructed so that a drum accommodating clothes is controlled to be rotated, thereby being capable of automatically carrying out steps of wash, rinse, dehydration and drying.
Furthermore, hot air is supplied into the drum in the drying step so that the clothes are dried.
Conventional drum type washing-drying machines are provided with an electric heater such as a sheathed heater or honeycomb heater serving as a heat source, whereby air is heated by the electric heater. Furthermore, the drum type washing-drying machine is provided with a heat exchanger which is adapted to dehumidify hot air which has already absorbed water from the clothes in the drum.
However, since the system of heating air by an electric heater consumes a large amount of energy, there is a possibility that the consumption of energy may result in a huge burden for a general household. A drum type washing-drying machine has therefore been suggested which employs a heat-pump mechanism to heat air. This type of drum type washing-drying machine is described, for example, in Japanese Laid-open Patent Application No. 2004-135715 (JP-2004-135715A).
The above-described heat pump mechanism comprises a construction circulating a refrigerant by a compressor into a condenser, capillary tube (throttle) and evaporator in this sequence. A hot-air circulation path is defined along an outer periphery of a water tub. The condenser and evaporator both constituting the heat pump mechanism are disposed in the circulation path. Hot air is then produced by heat exchange between air circulating through the circulation path and the condenser, whereby the hot air is dehumidified by the heat exchange with the evaporator.
However, when the aforesaid heat pump mechanism is employed, the hot air supplied into the drum unavoidably has a low temperature of about 60° C. or below, which temperature is approximately one half of a temperature in the use of an electric heater or lower. As a result, since the drying performance is lowered, a period of the drying step needs to be increased.
Furthermore, even a heat pump can exert substantially the same drying performance as achieved by an electric heater when a flow rate of recirculated air is rendered larger than in the use of an electric heater. For example, when a compressor has a cooling capacity of about 1500 W, a flow rate of circulating air needs to be set to about 3 m3/min. This value is twice to three times larger than a flow rate of circulating air in the use of an electric heater.
In order that such a high flow rate as noted above may be ensured, the blowing capacity of a blower fan needs to be increased, by rendering the blower fan large-scaled or by increasing a rotational speed of the blower fan. However, the large-scaled blower fan would increase an installation space of the blower fan, whereupon the size of a whole washing machine would be increased. Increasing the rotational speed of the blower fan would result in an increase in an amount of noise due to rotation of the blower fan.
Furthermore, the circulation air flow has an air-flow resistance proportional to the square of the air-flow velocity. Accordingly, the flow rate of the circulating air can be increased by increasing an area of the circulating air-flow path thereby lowering a mean flow velocity of circulating air. In this case, however, the circulating air-flow path is rendered larger, which results in an increase in the size of the whole washing machine.